This happened to me while I was doing some basic desktop work just a little while ago...

Nice strobe show in Windows 7 Ultimate (64 bits version)

Not sure what caused it exactly. I only know I had a webpage open in Firefox that's running from my local hard drive. Apparently it contained some sort of script that the system identified as a security risk. Upon closing that very tab everything started running smoothly again. Sensitive system, sensitive system indeed...

Not that I'm complaining or anything, though. I just sat back and enjoyed the nice visual extravaganza. Sort of like having a very, very lame stroboscope built into your operating system.